The UBV Photometric System, also called the Johnson System or Johnson-Morgan System, is an early well-accepted Photometric System and is the system to be assumed if no other system is specified or implied.
It specifies U, B, and V Bands, in turn, specifying specific B-V and U-B color indices. Magnitudes are calibrated as zero for A0 V Stars unaffected by Reddening.
The UBVRI Photometric System, i.e., Johnson-Cousins System is an extension with red and Infrared passbands, R Band and I Band.
The UBVI Photometric System uses the B, V, R, and I from the UBVRI system, but typically with a narrower U band from a system by Thuan and Gunn.